\documentclass{standalone}
\usepackage{tikz}
\usepackage{ctex,siunitx}
\setCJKmainfont{Noto Serif CJK SC}
\usepackage{tkz-euclide}
\usepackage{amsmath}
\usepackage{wasysym}
\usepackage{circuitikz}
\usetikzlibrary{patterns, calc}
\usetikzlibrary {decorations.pathmorphing, decorations.pathreplacing, decorations.shapes,}
\ctikzset{bipoles/length=1cm}
\begin{document}
\small
\begin{circuitikz}[>=latex, scale=1.0,european]
  % \useasboundingbox(0.9,0)rectangle(5.1,5);
  \draw (4,0) to[battery2] (3,0)--(0,0)--(0,2)to [rmeter,t=G](4,2)to [pR,name=hb](8,2)--(8,0);
  \draw(7.25,2)--(7.25,2|-hb.wiper)--(hb.wiper);
  \draw (4,0) to[cute open switch=$K_1$] (8,0);
  \node at (hb.center)[below=2mm]{$R$};
  \draw (0,1.0)to [cute open switch=$K_2$] (2,1.0) to [vR=$R'$](4.5,1.0)--(4.5,2);
  \draw (0,1.0) [fill=black] circle (1.2pt);
  \draw (4.5,2) [fill=black] circle (1.2pt);
  \draw (7.25,2) [fill=black] circle (1.2pt);
  % \node at (6,0)[below]{$K_1$};
  % \node at (1,1.5)[below]{$K_2$};
\end{circuitikz}
\end{document}